/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Register for a notification when the user clicks outside a specific widget.
[FDelegateHandle](API\Runtime\Core\Delegates\FDelegateHandle) RegisterClickNotification ( const [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > & NotifyWhenClickedOutsideMe, const FOnClickedOutside & InNotification )